What is sigil?

Sigil is a command-line tool that takes text containing placeholders and replaces them with actual values you provide. Think of it like a mail-merge: you write a template once with blanks like ${name} or {{ $port }}, and it fills in the blanks to produce a finished file. It supports two styles of placeholders. The first is POSIX-style variable expansion, familiar to anyone who has written shell scripts, things like $name or ${count:-3} (which means "use the value of count, or 3 if it's not set"). The second is a richer templating syntax using {{ }} that supports conditionals, loops, and modifier pipelines, so you can capitalize text, pull values from JSON files, include other templates, or repeat sections. You can pass variables as simple command-line arguments (name=Jeff) or load them from JSON, YAML, or .env files. The typical user is someone managing configuration files across different environments. For example, if you have a config template for a web server and need to generate different versions for development, staging, and production with different ports and database hosts, this tool handles that. It can even edit files in place, safely swapping in the rendered output only if processing succeeds. It also reads your environment variables automatically, which makes it handy for injecting secrets or settings into config files without hardcoding them. One notable design choice is that the templating engine is built on Go's standard template package, so anyone familiar with Go templates will feel at home. You can also customize the delimiter characters, which matters when your template itself generates text that contains {{ }}, like Packer configuration files, so the tool doesn't get confused about which braces to process and which to leave alone.